The present invention is directed to a battery pack and a battery module, and a method for operating a battery module.
It is known that the service life of lithium ion secondary battery cells is affected by excessive discharging and by overcharging. To prevent damage, these battery cells should be charged to a maximum voltage in the range of 4.0 to 4.5 V, and they should not fall below a voltage of 1.0 to 2.5 V.
If several cells are connected in a series connection to form a “battery pack”, it is not sufficient to monitor the voltage of the entire pack. Due to production-related scatterings of capacitance and parasitic discharge resistances, the cells are in various states of charge, which continue to drift apart from each other over the course of time, due to the charge/discharge cycles. An inhomogenous temperature distribution that occurs in the pack during operation also causes the battery voltages to drift. During charging, the cells in a pack therefore do not reach their end-of-charge voltage at the same time. This can result in individual cells becoming overcharged and, therefore, damaged. Conversely, when the pack is discharged, there is a risk that, when the end-of-discharge voltage of the pack is reached, individual cells will discharge below their permissible end-of-discharge voltage and become damaged. This can even go so far that, when the pack is discharged, the polarity of the cell is changed and the cell is destroyed, thereby rendering the entire pack unusable. Since asymmetries in the cells are amplified by the charge/discharge cycles, the case of the polarity of individual cells being changed occurs after a number of charge/discharge cycles that cannot be estimated in advance, resulting in the failure of the pack.
To prevent the service life of a battery pack from being shortened, it has already been proposed to monitor the cells in a pack individually. In this process, the voltage of each cell is monitored and, when a lower or upper limit is reached, the particular cell is bridged with compensation electronics. This can be accomplished using various suitable circuits. In any case, however, it is necessary for the positive and negative poles of each cell to be guided out of the pack.
Compensation electronics for cells in a pack are made known, e.g., in patent application GB 2408396 A. In that case, the connections of the cells are guided out via separated lines to the compensation electronics. These compensation electronics are located separate from the pack, which results in high fabrication costs, since the process of connecting the pack and the electronics is not automatable, and must be carried out manually. With mass-produced products such as battery packs in power tools, or planned series-production products such as hybrid vehicles, manual fabrication steps of this type are undesired, for reasons of time and costs.
Furthermore, it is problematic that cables may break or contacts may become detached when mechanical loads are applied that typically occur with power tools. Cable breaks or detached contacts may result in failure of the compensation electronics and premature failure of the battery pack, since it may no longer be properly monitored or charged.